2025-11-26 東京大学
図1：ハロー状に光るガンマ線放射の強度マップ
天の川銀河を中心とする銀河座標で表示。銀経は銀河面方向に測った経度、銀緯は銀河面から測った緯度。観測されたガンマ線強度マップから、ハロー以外の成分を除去したもの。銀緯±10度の銀河面領域は、天体起源の放射を避けるために解析から除かれた（中央灰色のバー）。
＜関連情報＞
- https://www.s.u-tokyo.ac.jp/ja/press/10983/
- https://iopscience.iop.org/article/10.1088/1475-7516/2025/11/080
銀河拡散放射の20 GeVハロー状超過と暗黒物質消滅への影響 20 GeV halo-like excess of the Galactic diffuse emission and implications for dark matter annihilation
Tomonori Totani
Journal of Cosmology and Astroparticle Physics Published: 25 November 2025
DOI:10.1088/1475-7516/2025/11/080
Abstract
Fifteen years of the Fermi Large Area Telescope (LAT) data in the halo region of the Milky Way (MW) are analyzed to search for gamma rays from dark matter annihilation. Gamma-ray maps within the region of interest (|l| ≤ 60°, 10° ≤ |b| ≤ 60°) are modeled using point sources, the GALPROP models of cosmic-ray interactions, isotropic background, and templates of Loop I and the Fermi bubbles, and then the presence of a halo-like component is further examined. A statistically significant halo-like excess is found with a spectral peak around 20 GeV, while its flux is consistent with zero below 2 GeV and above 200 GeV. Examination of the fit residual maps indicates that a spherically symmetric halo component fits the map data well. The radial profile agrees with annihilation by the smooth NFW density profile, and may be slightly shallower than this, especially in the central region. Various systematic uncertainties are investigated, but the 20 GeV peak remains significant. In particular, the halo excess with a similar spectrum is detected even relative to the LAT standard background model, which contains non-template patches adjusted to match the observed map. The halo excess spectrum can be fitted by annihilation with a particle mass mχ ∼ 0.5–0.8 TeV and cross section 〈συ〉 ∼ (5–8) × 10-25 cm3 s-1 for the bb̅ channel. This cross section is larger than the upper limits from dwarf galaxies and the canonical thermal relic value, but considering various uncertainties, especially the density profile of the MW halo, the dark matter interpretation of the 20 GeV “Fermi halo” remains feasible. The prospects for verification through future observations are briefly discussed.
